A constrained network and the nodes within them have may have limited packet sizes, may exhibit a high degree of packet loss, and may have a substantial number of devices that may be in a sleep mode (e.g., powered off) at any point in time but periodically “wake up” for brief periods of time. Nodes of constrained networks may be characterized by limits on throughput, available power, and/or RAM, and may be connected over a low power wireless connection (e.g., LoWPAN (Low-Power Wireless Personal Area Networks), such as IEEE 802.15.4).
In many applications that use IEEE 802.15.4, nodes are battery powered, and battery replacement or recharging in relatively short intervals may often be impractical. Therefore, power consumption may be of concern. Battery-powered nodes may often require duty-cycling to reduce power consumption. These devices may spend most of their operational life in a sleep mode; however, each node may periodically listen to an RF channel in order to determine whether a message is pending. This mechanism allows the application designer to decide on the balance between battery consumption and message latency. IEEE 802.15.4 and 802.15.5 define power saving techniques at the Media Access Control (MAC) and Physical (PHY) layers in constrained networks.
IEEE 802.15.4 defines two ways of synchronization between nodes, both with and without beacons. The standard sets forth that all devices operating on a beacon-enabled Personal Area Network (PAN) shall be able to acquire beacon synchronization in order to detect any pending messages or to track the beacon. Further, all devices operating on a non-beacon-enabled PAN shall be able to poll the coordinator for data at the discretion of the next higher layer.
IEEE 802.15.5 defines a Synchronous Energy Saving (SES) power saving mechanism that utilizes a synchronous time schedule for data transmission. For SES, all mesh devices should be synchronized network-wide and have the same time structure including an active and an inactive duration. A wakeup notification command frame allows a mesh device to notify its own schedule information to neighbors in the optional energy saving mode. When a setting (e.g., meshASESOn) is set to TRUE, it should be transmitted at the beginning of the active duration for every wakeup interval.
Constrained Application Protocol (CoAP) is an HTTP-like application layer protocol designed for constrained networks. The protocol is REQUEST/RESPONSE based and uses the Representational State Transfer (REST) architecture. CoAP defines at least 4 HTTP-like methods, for example GET, POST, PUT and DELETE.